`train_walk_forward` called `reset_for_fold().await?` directly at the
fold boundary, which applies `shrink_and_perturb(α=0.8, σ=0.01)` to
whatever `params_flat` holds. At end-of-fold, `params_flat` carries the
LATEST-EPOCH decayed weights, NOT the best-Sharpe snapshot saved
mid-fold. Fold N+1 then started from `0.8 × decayed + noise` and
carried the within-fold edge-decay forward.
Insert `restore_best_gpu_params()` BEFORE `reset_for_fold` so S&P
operates on the best-Sharpe snapshot. Cold-start guard swallows the
"no snapshot saved" error on the first fold (or any fold without a
Sharpe improvement) — matches pre-fix behavior on cold start, no
regression.
State interaction with `reset_for_fold`: restore-best writes
`params_flat` ONLY (online weights); reset_for_fold then runs S&P on
the restored online, hard-syncs target ← restored online, and resets
Adam state on every optimizer. Non-conflicting.
`best_params_snapshot` is constructor-init `None` on FusedTrainingCtx
and is NEVER cleared at fold boundary. The trainer's `self.best_sharpe`
IS reset to NEG_INFINITY in `reset_for_fold` so the first improvement
in fold N+1 will overwrite the snapshot. Until then, the snapshot holds
whichever fold most recently saved a peak — intended training-scoped
behavior. Strict within-fold semantics flagged as a follow-up
consideration.
Behavioral test (CPU oracle) pins the math contract:
- shrink_and_perturb(α, σ) on X = α × X + (1−α) × N(0, σ)
- with-fix vs without-fix differ by α × (W_best − W_curr)
- cold-start (best == current) is bit-identical between orderings
GPU-level kernel coverage stays in compile_training_kernels smoke and
the L40S 30-epoch validation gating SP18 Phase 1.
Pre-commit Invariant 7: docs/dqn-wire-up-audit.md updated with full
rationale, state-interaction analysis, lifecycle notes, and the
preserved cross-pearl invariants.
Per:
- feedback_no_partial_refactor (call-ordering + test + audit-doc atomic)
- feedback_no_legacy_aliases (reuses existing API unchanged)
- feedback_wire_everything_up (restore_best_gpu_params gains second
cold-path consumer)
- pearl_no_host_branches_in_captured_graph (runs outside graph capture)
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